AmbientLight

AmbientLight red, green, blue

Sets the ambient light, that will affect every object on the scene, from all sides.

It's possible to set a value for every color component: for example,

AmbientLight 255,0,0

Makes the light red; instead,

AmbientLight 0,0,0

will turn off the ambient light: so, unless there are other lights, the scene will be black, and no object will be visible.

The ambient light is, by default, at 127,127,127; so, it will make possible to see on the scene, even if there aren't other lights; anyway, since the light comes from every side, there will be no shading, and objects will look "flat".